We have found granulosa cell tumors in young females of two families of SWR/J mice. We plan a breeding program to increase and fix the incidence of these tumors in a subline to be designated SWR/WT. We hope to determine the genetic basis of the inheritance of the tumor. We will establish lines by transplantation and by culture, and study the morphology and endocrinology of spontaneous and derived tumors. Attempts will be made to determine the histogenesis of the tumor by the study of young and fetal animals and by the transplantation of embryonic structures. The strain SWR/Wt will be used as a model to study the treatment of and the prevention of recurrences of granulosa cell tumors.